A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depth of features in semiconductor devices are decreasing. State of the art semiconductor devices require junction depths less than 1,000 angstroms and may eventually require junction depths on the order of 200 angstroms or less.
Ion implantation is a standard technique for introducing conductivity-altering dopant materials into semiconductor wafers. In a conventional ion implantation system, known as a beamline ion implanter, a desired dopant material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are imbedded into the crystalline lattice of the semiconductor material.
Plasma doping systems may be used for forming shallow junctions in semiconductor wafers. In a plasma doping system, a semiconductor wafer is placed on a conductive platen which functions as a cathode. An ionizable gas containing the desired dopant material is introduced into the chamber, and a voltage pulse is applied between the platen and an anode or the chamber walls, causing formation of a plasma having a plasma sheath at the surface of the wafer. The applied voltage causes ions in the plasma to cross the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode.
The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Shallow junctions are obtained with low implant energies. However, the annealing process that is used for activation of the implanted dopant material causes the dopant material to diffuse from the implanted region of the semiconductor wafer. As a result of such diffusion, junction depths are increased by annealing. To counteract the increase in junction depth produced by annealing, the implant energy may be decreased, so that a desired junction depth after annealing is obtained. This approach provides satisfactory results, except in the case of very shallow junctions. A limit is reached as to the junction depth that can be obtained by decreasing implant energy, due to the diffusion of the dopant material that occurs during annealing. In addition, conventional ion implanters typically operate inefficiently at very low implant energies.
In addition to shallow junction depths, implanted regions are required to have low sheet resistance for proper operation of the devices being fabricated on the semiconductor wafer. The sheet resistance depends in part of the effectiveness of the activation process.
Furthermore, implanted regions are required to have low damage in order to achieve low leakage current devices on the semiconductor wafers. Typically, damage has been removed during the annealing process. However, as noted above, other problems are produced by the annealing process. These factors have presented difficulties in achieving ultrashallow junctions which have low sheet resistance and low damage.
Accordingly, there is a need for methods for fabricating in semiconductor wafers ultrashallow junctions which have low sheet resistance and low damage.